use strict;

# define hash keys that need conversion
# (others will be unchanged)
my %map_foo_to_bar = (
    gearbox    => 'gearbox1',
    drivetrain => 'the_drivetrain',
    smokestack => 'my_smokestack',
    junkyard   => '_junkyard',
);

# our hash data
my %foo = (gearbox        => 'five-speed', 
           drivetrain     => 'automatic', 
           something_else => 'entirely');

# perform the mapping (wrapped in an anonymous hashref)
my $new_name;
my $bar = { 
  map 
    { defined ($new_name = $map_foo_to_bar{$_}) ? 
      ($new_name => $foo{$_}) : 
      ($_        => $foo{$_})
    } 
    (keys %foo) 
  };